The present invention concerns an improved printing paper which has the appearance of uncoated paper and printability properties approaching those of coated paper, a paper surface treatment formulation, and a method for applying the formulation to produce the improved printing paper.
It has been common practice to apply aqueous coatings, consisting of pigments, binders/adhesives, and various functional additives, to paper to produce printing paper. The coatings cover the fibrous paper sheet surface and create an interface for printing inks that is smoother and less absorbent than the uncoated surface.
Prior to coating, the uncoated paper web exhibits many xe2x80x9cmicroxe2x80x9d depressions. The topography of the micro depressions can be seen in FIG. 1. The micro depressions are often the result of open areas between individual fibers and fiber bundles in the paper. The dimensions of these open areas determine the roughness of the paper surface and the amount of coating required to completely fill in the depressions. The micro depressions on the rough sheet surface can vary in depth from as little as 5 microns to as much as 25 microns (1 micron=1xc3x9710xe2x88x926 inch). The average depth of the micro depressions is 10 microns.
The bulk-to-weight ratio of typical coating layers in the prior art is approximately 1.00 micron per lb. of coating of applied, per side, per ream of paper. As used herein a xe2x80x9creamxe2x80x9dequals approximately 3,300 ft2 of paper. Application of one lb. of conventional coating per side, per ream of paper will generally result in a layer of coating on the surface of the paper that is 1 micron thick.
FIG. 2 illustrates the topography of a typical paper sheet surface when a coating is applied to the surface by a blade coater, a coater commonly used for applying aqueous coatings for the production of printing paper. To completely fill in the micro depressions on the surface of a paper web, prior art processes will apply 5-25 lbs. of coating per side, per ream of paper web. The amount of coating needed will depend on the formation of the paper web, its absorbent properties, and on the roughness of the particular paper web. For instance, 25 lbs. of coating per side, per ream of paper web may be needed to completely fill in the micro depressions on the surface of a very rough, absorbent paper web.
Although greatly improving print properties, the amount of coating used in coated papers of the prior art significantly alters the appearance of the paper. Coated papers, whether glossy or matte in finish, have an appearance that is much different than that of uncoated premium papers, such as text and cover paper. This difference is considered undesirable by many paper consumers who prefer the appearance of uncoated paper but need the printability properties of coated paper.
Uncoated paper typically can be lightly coated, but the amount of coating is less than 5 lbs. per side, per ream of paper. This lower amount of coating does not alter the desirable uncoated appearance of the paper. However, prior art uncoated papers of this type have never achieved the printability properties of coated paper. Thus, there is great interest in the development of a printing paper which combines the desirable appearance of uncoated paper with the printability properties of coated paper.
Several techniques have been developed in attempts to utilize existing size press equipment to manufacture coated paper. Size presses are typically used in the art to make uncoated paper and are normally used to apply coatings having low viscosities of less than 100 centepoise (xe2x80x9cc.p.s.xe2x80x9d) (Brookfield, 100 r.p.m.). One such technique, known as xe2x80x9cfilm coatingxe2x80x9d or xe2x80x9cwash coating,xe2x80x9d utilizes existing size press equipment to achieve a slight improvement in the printing paper over uncoated offset paper grades without the capital expense of normal coating treatments. The resulting paper is, however, a lower quality coated paper and often exhibits undesirable characteristics, such as mottle (galvanized gloss) and roll film split pattern.
Another technique involves the use of micro-spherical particle pigment, which has been used to promote gloss and opacity in coated paper. For instance, Kowalski et al. (U.S. Pat. No. 4,427,836) relates to a process for manufacturing water-insoluble, heteropolymer, micro-spherical particles, which may be used as an opacifier or thickener when incorporated in a paint or on aqueous coating.
Vassiliades, et al. (U.S. Pat. Nos. 3,816,169 and 3,822,181) disclose spherical microcapsules that may be used as an opacifier when incorporated in or coated on a paper web. These patents concern the opacifying and/or brightening of the paper web with no regard for printability. The patents do not disclose any specific formulations that could be used to coat the paper. Nor do the patents discuss how such formulations could be applied.
Rohm and Haas Company manufactures Ropaques(copyright) a pigment for use on paper, which is composed of polymeric, hollow, spherical particles ranging in diameter from 0.3-1.0 micron. These particles enhance printability of paper and have a high bulking capability. The particles may be combined with typical binders, such as starch binders, and other ingredients, to create coatings having a greater thickness at a lower weight than typical coatings. When these coatings are applied at low viscosity (less than 100 c.p.s., Brookfield, 100 r.p.m.) using the xe2x80x9cwashxe2x80x9d or xe2x80x9cfilmxe2x80x9d coating techniques described above, the resultant paper exhibits a marginal improvement in printability and opacity.
The invention provides a printing paper that has the appearance of uncoated paper but surprisingly has improved printability properties approaching those of conventionally coated papers, which result from the application of 5-25 lbs. of coating per side, per ream of paper. The paper of the present invention achieves approaching those of conventionally coated papers with the application of a suitable surface formulation at weight levels substantially less than those used in conventional coating methods.
The printing paper of the invention comprises a paper web and a layer of surface formulation applied to and dried on each side of the paper web in an amount of approximately 2-5 lbs. of formulation per side, per ream of paper web. The paper has improved printability properties approaching those of coated paper, yet retains the appearance and desirable characteristics of uncoated paper. Papers of the invention are characterized by an opacity in the range of about 95-100%, an ash content of about 5-25% by weight, a Sheffield smoothness of about 25-250 cc/min., and a Gurley density of about 200-4,000 seconds. These papers also possess the additional characteristics of a brightness of up to about 99%, a stiffness of about 150-4,000 grams, and a KandN drop percentage of about 10-20%. This combination of paper characteristics has never been achieved in a printing paper prior to this invention. A preferred embodiment of the invention relates to text and cover printing papers having the characteristics set forth above.
The papers of the invention have an increased bulk-to-weight ratio of the layer of formulation on the paper surface. The bulk-to-weight ratio of the layer of formulation on paper according to the invention is greater than about 1.15 microns per lb. of formulation applied, per side, per ream of paper web. This bulking characteristic also has never been achieved in a printing papers prior to the invention.
The improved characteristics of the paper of the invention are achieved through the use of a novel surface treatment formulation which is applied to the surface of the paper web. The formulation comprises a pigment composed of hollow particles, and a high molecular weight modified starch binder exhibiting suitable rheological properties.
The hollow particle pigment promotes printability properties in the paper. In addition, the pigment affords increased bulk of the formulation dried on the paper surface with minimum weight application of formulation. In a preferred embodiment of the invention, the hollow particles are spherical and constructed of a synthetic polymer.
The binder is typically a high molecular weight modified starch having optimal rheological properties that make it suitable for application in the methods of the invention. xe2x80x9cModifiedxe2x80x9d starch binders are starch binders that have been altered chemically to control their molecular weight and/or chemical structure, hence changing their viscosity characteristics after cooking. A number of methods of modifying starch and the molecular weights of modified starches are well known in the art.
It has been found that modified high molecular weight starch binders are particularly useful, because the rheological properties of the binder, when combined with the pigment, provide the formulation with a variable viscosity, which is low enough so that the formulation flows as a liquid when agitated, but which increases quickly when the formulation is not agitated or when it is cooled. In preferred embodiments, the binder is a high molecular weight modified cationic potato starch binder. As is common in the art, the skilled artisan may also add other additives to the formulation, such as print aids, anti-scuffing agents, sizing agents and defoamers to achieve various properties.
When agitated, the surface treatment formulation of the invention typically has an initial viscosity of approximately 2,000-4,000 c.p.s. (Brookfield, 100 r.p.m.), preferably 3,000 c.p.s. (Brookfield, 100 r.p.m.). In this range, the formulation is quite thick, appearing similar to a batter, but it will flow as a liquid, and can be pumped through the plumbing of typical coating machines. The formulation is supplied to a coating applicator when it is in this viscosity range. The applicator applies a layer of formulation on the surface of a paper web.
Upon application of the formulation to the paper web, the initial viscosity level of the formulation prevents the formulation from soaking or de-watering into the paper web. In addition, the viscosity of the formulation quickly increases upon application, to further inhibit the formulation from soaking or de-watering into the paper web. The formulation may also be cooled prior to being applied to the paper web surface to further quicken the increase in viscosity and hence further inhibit the formulation from soaking or de-watering into the web. Through these properties of the formulation, a maximum amount of formulation will remain on the surface of the paper after application. In a preferred embodiment, the viscosity of the formulation applied on the surface of the paper web will increase rapidly to greater than 8,000 c.p.s., and even more preferably to greater than 10,000 c.p.s. (Brookfield, 100 r.p.m.).
The maximization of the formulation on the paper surface increases the ultimate bulk-to-weight ratio of the formulation layer when it dries on the surface. Thus, the bulk-to-weight ratio of formulation layer on the surface of the paper according to the invention is higher than the bulk-to-weight ratio of coating layers in the prior art. The layer of formulation in the present invention has a bulk-to-weight ratio of greater than about 1.15 micron per lb. of formulation applied per side, per ream of paper. The preferred bulk-to-weight ratio of the formulation layer is about 1.40 micron per lb. of formulation per side, per ream.
The formulation is preferably applied to each side of the paper web by a contour type metering applicator, such as a high speed metering applicator, roll coater, etc., in a substantially uniform thickness over the topography of the paper surface. This application process minimizes the amount of formulation needed to completely cover the surface of the paper regardless of the roughness of the surface, and aids in achieving the desired printability properties. The formulation of the instant invention may be applied in much lower amounts than typical coatings. The preferable amount of formulation is about 2-5 lbs. per side, per ream of paper. This amount of formulation improves print results substantially without significantly altering the uncoated appearance of the paper.
The paper prepared using the formulations and methods of the invention has increased lithographic ink efficiency (increased density of color with less ink applied), improved print sharpness (less % dot gain), improved ink gloss, and decreased ink show-through, yet the paper has an uncoated appearance. The resultant paper is also improved in that it is stiffer than typical uncoated text and cover papers. Finally, in its calendered state, the resultant paper offers optimum print results from current digital print technology due to its superior smoothness compared to prior art uncoated papers.